On Friday, the QLine streetcar system begins rolling along a 6.6-mile loop from downtown to New Center on the city’s fabled Woodward Avenue.

Detroit joins at least 15 U.S cities that have launched a streetcar system since 2000. Detroit, like every other city, believes streetcars are economic engines that will overhaul struggling areas by linking them to more vibrant sections. They can dramatically increase foot traffic, which leads to more businesses and housing, according to data from multiple cities, including Cincinnati, Houston, Washington and Minneapolis, and essentially give an area a hip, big-city feel.

“QLine has already spurred billions of dollars of investment with billions more to come,” wrote Dan Gilbert, chairman and founder of Quicken Loans, in an email to The Detroit News. “It is more than a transportation machine, it is a jobs-creating machine.”

Quicken bought the naming rights to Detroit’s streetcar line.

The debate over whether streetcars work never seems to end in many cities. They are a political litmus test for local candidates. They are used by both sides in the gentrification battle, based on ongoing arguments in Atlanta, Oklahoma City, Tampa and other U.S. cities.

But the streetcar trend is accelerating — for now. The Motor City is among five cities since 2016 to have debuted their streetcar services.

Based on what’s happened in other cities, here is a mix of street-level challenges and some of the broader debates that likely will accompany the QLine:

■Vehicles and people will get in the way. Detroiters are not used to sharing the road with a massive streetcar that can’t stop as quickly as a car, truck or bus. Various cities report an average of 10 accidents a month in the first three months of service, including the occasional fatal wreck. The accident rate tends to go down year after year, based on data collected in such cities as Dallas, Portland and Houston. Federal statistics show streetcars have the lowest rate of accidents among any form of rail mode, according to the Federal Transit Administration.

The QLine has three lane changes along Woodward, which could be potential trouble spots.

“How is this going to work? I don’t know,” said Kimani Brandon, 26, as he recently watched a QLine streetcar wind its way through the Campus Martius roundabout, which caused a small traffic jam as vehicles behind it slowed. Brandon, who works downtown, said he hopes to take the QLine often to go to Midtown and New Center.

“I want it to be great, but, we’ll see,” he said. “It just seems like there’s going to be a learning curve for a lot of people how to drive around it.”

Brandon realizes he can take a public bus to get to Midtown and New Center from downtown but the “wait can be pretty long.”

The Detroit native said he took the bus often when he was younger.

“I’m thinking with the shorter route, the QLine might be different,” he said.

Vehicles parking on the tracks, especially delivery trucks, are often an issue. QLine officials are advising motorists who park along Woodward to park within the marked white parking lines or risk having their vehicles towed. In the initial months, several cities — including Kansas City, Cincinnati, Washington and Kansas City — cracked down on vehicles on the tracks by towing them quickly.

Pedestrians need education, too. Pedestrians should step over, and not on, the tracks when they cross the street, QLine officials advise. The tracks are lubricated to cut down on the streetcar’s noise level.

■The number of people who will ride the QLine may be hard to predict. Most cities wrongly forecast ridership. At least six cities overestimated while at least three underestimated. Cold months can chill ridership numbers and big events such as concerts or sporting events tend to clog the line.

QLine streetcars are three-piece, articulated cars that are expected to carry an average of 125 passengers per car. Four cars will operate at most times. At peak hours and during special events, QLine officials say they will activate more cars.

■Fans, critics and boosters only seem to grow more convinced over time they are right. It appears Detroit will be no different. QLine officials say the line has already sparked $7 billion in investment along Woodward and surrounding streets since 2013.

“More real estate investment will follow, I guarantee it,” said Rip Rapson, president and CEO of the Kresge Foundation. The philanthropic group is the largest individual funder of the QLine, contributing $50 million.

Others, such as Progress Michigan, say the investment angle proves the line is built to benefit mainly big businesses and not people.

“It is plain to see that this rail is a private venture, hijacked by private dollars, and is not meant to support rebuilding Detroit,” wrote Sean Tobin, data and policy analyst for Progress Michigan, on the political watchdog group’s website.

Nevertheless, the excitement is real among many business owners and others along the QLine, and in most cities, investment increases once the streetcars go online. Detroit Hardware owners Emily Webster and Anna Sparkman said anticipation of the QLine has already transformed a stretch of Woodward in New Center.

“We’ve already seen so much ownership change of properties and places being fixed up,” Webster said.

■Most cities want their streetcar lines to expand. Despite the glitches and the entrenched critics, at least six cities have already expanded their initial streetcar lines. QLine officials and boosters already are advocating the time is now to plan to keep the streetcar line growing.

Matt Cullen, CEO of M-1 Rail, which owns and operates the QLine, said the streetcar will prove Metro Detroit can embrace mass transit and could lead to expansion.

“We hope over time it will expand north along Woodward,” Cullen said. “And in the next 10 years, there may be a couple of new lines possibly along Jefferson or Gratiot.”